Method and device for continually producing a suspension of cellulose in an aqueous amine oxide

ABSTRACT

The invention relates to a method for continually producing a suspension of cellulose in an aqueous tertiary amine oxide. The method includes (a) mixing cellulose and an amine oxide-free aqueous phase to form a cellulose suspension in a mass ratio in the range of 1:3 to 1:40, (b) dehydrating the cellulose suspension, wherein the dehydrated cellulose suspension has a cellulose content in the range of 20 to 80 percent-by-mass; (c) mixing the cellulose suspension with an aqueous amine oxide while the cellulose suspension is falling through a vertical fall zone to obtain an aqueous amine oxide-cellulose suspension with an amine oxide content in the range of 70 to 80 percent-by-mass; and (d) conveying the aqueous amine oxide-cellulose suspension through a horizontal shear zone. The invention also relates to a device for carrying out this method.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is filed under the provisions of 35 U.S.C. §371 andclaims the priority of International Patent Application No.PCT/DE00/03413 filed Sep. 29, 2000, which in turn claims priority ofGerman Patent Application No. 100 13 777.6 filed Mar. 20, 2000.

BACKGROUND OF THE INVENTION

1. Field of Technology

The invention relates to a method for the continuous preparation of acellulose suspension in an aqueous tertiary amine oxide for use in theLyocell method. The invention also relates to a device for performingsaid method.

2. Description of Related Art

It is known to prepare cellulose form- and spinning masses fromcellulose solutions in amine oxides, preferably inN-methylmorpholine-N-oxide and a non-soluble agent for cellulose,preferably water. By forming the solution and drawing and regeneratingthe cellulose, products are obtained for diverse use in the textile- andnon-textile area (see W. Berger, Options and Limits of AlternativeCellulose Dissolution and -Drawing, Lanzinger Reports 74 (1994) 9, pages11 to 18). BISFA assigned the generic term “Lyocell” to fibers producedwith this method.

WO 94/28217 discloses a discontinuous method for the preparation of acellulose suspension in aqueous amine oxide, where comminuted celluloseand an amine oxide solution are mixed by a rotor with radial stirringelements in a horizontal mixing chamber. The duration per batch isstated as 21 minutes. This method of operation is disadvantageousbecause two such mixing chambers have to be operated in parallel due tothe continuous load of the subsequent dissolution step. Furthermore, thecomplete emptying of the mixing chambers leads to difficulties.

It is also known from WO 96/33221 to prepare a cellulose suspension inaqueous N-methylmorpholine-N-oxide (NMMO) by mixing the comminutedcellulose in an annular layer mixer directly with the aqueous NMMO, suchas 75 percent-by-mass NMMO. The formed suspension is brought todissolution in a separate film extruder. The disadvantage of the annularlayer mixer is that only comminuted, essentially dry cellulose can beused. If the cellulose contains water, the formation of layers in themixer is difficult due to the mixing with NMMO solution, which was addedseparately. The water has to be separated thermally. The solublecomponents of the cellulose reach the spinning solution and lead todisadvantageous properties in the cellulose products. Because thesuspension is transported as a layer, the throughput relative to thecross-section of the apparatus is low.

From DE 198 37 210.8, it is known to suspend the cellulose in waterprior to the formation of a suspension in amine oxide solution, and topartially separate it again from the suspension agent after a certainamount of time. Then the wet cellulose material is first conveyedthrough a horizontal shear zone in the absence of amine oxide, and thentogether with amine oxide. This method of operation requires arelatively long shear zone and thus a significant effort with respect toapparatus. The formation of a uniform suspension is complicated by thefact that the aqueous oxide is fed into a cellulose material thatcompletely fills the available volume of the apparatus.

The present invention is based on the problem to provide a method and adevice for the continuous preparation of a cellulose suspension in anaqueous tertiary amine oxide with a reduced apparatus effort for theformation of suspensions and the activation of cellulose for use in theLyocell process. Especially, the throughput of the cellulose materialrelative to the volume of the shear zone is supposed to be increased,and thus the preparation time of the suspension is supposed to bereduced. Other advantages are shown in the following description.

Thus, the invention is based on a method for the continuous preparationof a cellulose suspension in an aqueous tertiary amine oxide for use inthe Lyocell process, where (a) a cellulose suspension is formed fromcellulose and an amine oxide-free phase in a mass ratio in the range of1:3 to 1:40, (b) the cellulose suspension is dehydrated into a materialwith a cellulose content in the range of 20 to 80 percent-by-mass, and(c) the wet cellulose material is mixed with enough aqueous amine oxideand conveyed through a horizontal shear zone so that after the mixing, asuspension with an amine oxide content in the liquid phase in the rangeof 70 to 80 percent-by-mass is obtained, whereby the suspensionessentially completely fills the available conveying diameter in theshear zone.

In accordance with the invention, this method is characterized in thatin step (c), aqueous amine oxide in finely dispersed form is added tothe wet cellulose material in a fall zone only partially filled with thecellulose material, and the cellulose material mixed with the amineoxide is introduced into the shear zone. In a variant of the methodaccording to DE 198 37 210.8, the first part of the shear zone, wherethe cellulose material is conveyed and homogenized in the absence ofNMMO, is obsolete. Furthermore, the aqueous amine oxide is notintroduced into a shear zone that is essentially completely filled withthe suspension, but rather only into the upstream fall zone. Thecellulose material fills the fall zone only partially, so that theintroduction of the amine oxide simultaneously leads to apre-distribution of the amine oxide in the wet cellulose material. It issaid pre-distribution of the amine oxide that allows a shortening of theshear zone for the formation of a uniform suspension for the subsequentformation of the solution, which follows the fall zone. Compared to themethod of operation according to DE 198 37 210.8, this results in ashortening of the shear zone by approximately one third, and a reductionof investment costs by approximately 10 to 15 percent.

According to the preferred embodiment of the method in accordance withthe invention, the aqueous amine oxide is injected into the fall zone.In that way, the falling particles of the wet cellulose material areenveloped by the aqueous amine oxide and a good pre-mix is obtained,which allows a shortening of the subsequent homogenization frame in theshear apparatus. Preferably, N-methylmorpholine-N-oxide-monohydrate isinjected into the fall zone.

Surprisingly, it was found that the quality of the pulp was improved,despite the reduction of the process steps and/or the production time ofthe suspension, which can be determined by means of the very goodswelling condition of the cellulose particles in thethree-substance-mixture cellulose/NMMO/water at the end of the shearzone. This is attributed to the fact that the fine dispersion of theamine oxide in the fall zone shortens the time period otherwise requireduntil the NMMO solution acts on the cellulose particles, so that theoverall swelling of the particles has already progressed farther at theend of the shear zone. The quantity of the aqueous amine oxide added inthe fall zone is coordinated with the starter cellulose quantity suchthat the amine oxide content in the aqueous phase of the formedsuspension is in the target range of 70 to 80 percent-by-mass.

SUMMARY OF THE INVENTION

In the aforementioned step (a), the supplied cellulose material (inroll- or sheet form) can be broken down with water in a pulper orhomogenizer. In this step, the reactivity and/or accessibility of thecellulose chains for the amine oxide can be improved with the additionof enzymes. The dehydration in step (b) can be achieved by means ofbelt-type press, screw compactor, roller filter or centrifuges.Preferably, the dehydration is continued until the cellulose content isin the range of 45 to 55 percent-by-mass. The water content, which stillfluctuates slightly following dehydration, can be equalized duringtransport to the weight scale. Because the aqueous amine oxide that isused is frequently a regenerate solution that was reclaimed from a usedprecipitation bath, the regeneration of the precipitation bath can focuson the formation of an amine oxide concentrate in the requiredconcentration. The short-term shearing in a short shear zone achieves asufficient size reduction and homogenization of the cellulose particlesthat is not possible in the known ring layer mixer. Generally, it ispossible in accordance with the invention to use an aqueous amine oxidewith a mol ratio of amine oxide to water in the range of 1:1 to 1:2.2.The most concentrated amine oxide, i.e., especially NMMO monohydrate, isused with aqueous cellulose that was broken down with water, forexample, prior to the suspension in accordance with the invention,and/or was pre-treated enzymatically or thermally.

DETAILED DESCRIPTION OF THE INVENTION

The preparation of the pulp is appropriately performed at a temperaturein the range of 75 to 100° C. At these temperatures, the reduction ofthe cellulose and the breakdown of the amine oxide is low; on the otherhand, the increased temperature favors the homogenization and theuniform mixing of the wet cellulose and the amine oxide. The temperedsuspension can be brought to solution without any significant change oftemperature in a subsequent step by water evaporation under vacuum.

BRIEF DESCRIPTION OF DRAWINGS

The illustration shows schematically a facility for performing themethod in accordance with the invention.

According to the preferred embodiment of the method in accordance withthe invention, the pulping is performing in a continuous run and thesuspension is then pulled off continuously at the end of the shear zoneand introduced into the dissolution step without the need for anintermediate container.

Preferably, work in the shear zone proceeds with a standing time in therange of 5 to 30 minutes, especially in the range of 10 to 15 minutes.

The method can be performed with the addition of additives, such asstabilizers. Said stabilizers can also be used in the fall zone,together with the amine oxide.

The invention is furthermore based on a device having (a) a mixingapparatus with mixing organs, feed nozzles for cellulose and aqueoussuspension agent and discharge nozzles for the suspension, (b) aseparation apparatus connected to the mixing apparatus for the partialseparation of the suspension agent from the cellulose, and (c) a shearapparatus with horizontal shafts equipped with shearing tools, a feedsystem for the cellulose material from the separation apparatus at theone end of the shear apparatus and a discharge nozzle for the suspensionat the other end of the shear apparatus.

In accordance with the invention, this device is characterized in thatthe charge system is a fall pipe for forming a fall zone, and hasnozzles directed into its cavity to introduce the aqueous amine oxideinto the cellulose material. While the charge system of the shearapparatus according to DE 198 37 210.8 is a funnel to introduce thecellulose material, which has the form of bulk material, into the shearapparatus, the charge system in accordance with the invention isprovided to form a fall zone through which the particles of thecellulose material fall, and which therefore offers the possibility tospray these particles with the aqueous amine oxide so that a pre-mix ofthe wet cellulose material and the aqueous amine oxide is createdalready in the fall zone, and said premix is drawn as such into theshear apparatus. The charge system can be a vertical pipe that narrowscylindrically or conically toward the shear apparatus. When said premixenters the shear apparatus, the mixing of the cellulose particles withthe amine oxide has already progressed farther than when the amine oxideis introduced into the shear apparatus with the method in accordancewith DE 198 37 210.8. Thus, it is possible to shorten the length of theapparatus significantly without any negative impact on the swellingcondition of the cellulose particles in the suspension.

Preferably, at least one loop line equipped with nozzles is arranged inthe charge system. The amine oxide is injected into the fall pipethrough said loop line. In that way, it is of advantage that theparticles or aggregate particles of the wet cellulose material fallingthrough the pipe are as small as possible. If desired, the partiallydehydrated cellulose material can be guided through a crusher prior toentering the fall pipe.

Appropriately, the loop line, of which there is at least one, isconnected to a heated supply line. This prevents the aqueous amine oxidefrom solidifying in the lines and nozzles.

The shear apparatus can be a horizontal, multiple-shafted reactor suchas a multi-screw dissolver with self-cleaning heat exchange surfaces, areactor with 2 to 8 engaging screws that rotate in the same direction,or a multi-chamber mixer.

The invention is explained in greater detail in the following by meansof the illustration and an example.

The illustration shows schematically a facility for performing themethod in accordance with the invention. A mixing container 1 iscontinually charged with cellulose through the nozzle 2 and with wateras a suspension agent through the nozzle 3. The aqueous cellulosesuspension formed in the container 1 travels through the line 4 to abelt-type press 5, where it is dehydrated to a liquid content of 50%.The wet cellulose material obtained on the belt-type press travels asfleece through a weight scale 6 and reaches a funnel-shaped fall-pipe 7,which has, in the shown embodiment, an internal loop line 8 with nozzles9 directed into the inside of the pipe. The loop line 8 is charged withaqueous NMMO through a heated supply line 10. When the wet cellulosematerial falls through the pipe 7, it is wetted and charged with theinjected aqueous amine oxide so that a mixture of aqueous cellulosematerial and amine oxide is created at the lower end of the fall pipe 7,which also contains air cushions. Connecting to the fall pipe 7 is amultiple-shaft apparatus 11, where the mixture drawn into the apparatusis mixed further by the shearing—and conveyer elements (not shown)arranged on the shafts 12, and is then conveyed to the discharge pipe13. The air drawn into the fall pipe 7 with the mixture and some of thewater vapor are drawn off through the nozzle 14. A dissolution apparatusconnects to the discharge pipe 13. Said dissolution apparatus isconstructed similar to the multiple-shaft apparatus 11. The formation ofthe cellulose solution takes place in said dissolution apparatus. Theapparatus 11, as well as the dissolution apparatus (not shown) have aheating blanket (not shown) to maintain the desired mixing- ordissolution temperature.

EXAMPLE

In a turbo dissolver, 70 kg of a spruce sulfite cellulose material(Cuoxam-DP 510; α-cellulose content>90%) are broken down and homogenizedwith soaking liquid in a liquor ratio of 1:20. The suspension is pumpedinto a supply tank with the help of a slush pump and diluted to a pulpconsistency of 10 grams per liter at a temperature of 50° C. On a wetfleece arrangement with subsequent lump breaking, a cellulose fleecewith a solid matter content of 50% is prepared and continuallyintroduced into a double-screw apparatus via a fall duct at a throughputof 33.5 kg of cellulose per hour. The homogenization of the cellulosetakes place during the transport to the weight scale. Over a heatedsupply line, NMMO-monohydrate is added simultaneously in a quantity of127.3 kg per hour through nozzles in the fall pipe. In the subsequentshearing and homogenization zone of the double-screw apparatus, thecellulose/water suspension is mixed uniformly with the NMMO monohydrate.A pulp with a 76% NMMO content is obtained, and a spinning solution with12.9% cellulose content can be prepared in a subsequent evaporizationstep.

In the quality assessment of the suspension following the pulper, theswelling condition of the cellulose particles was measured according tothe ASG standard. A maximum of 3 non-swelled cellulose particles per cm³were determined in the pulp, which, according to the quality standard,meets the rating “very good.”

The quality evaluation of the spinning solution formed from thesuspension was performed in the V 300 microscope by Hund, Wetzlar withan evaluation unit (JVC video camera and printer). The number ofundissolved cellulose particles of the spinning sample is stated as 1cm³ each. The following quality categories apply:

No. of undissolved cellulose particles/cm Rating 0 to 5 1  6 to 10 2 11to 15 3 15 not spinnable

The spinning solution of this example did not contain any undissolvedparticles/cm³. The cellulose solution is well suited for the directprocessing into fibers, filament yarns and sheets according to thedry-wet spinning process.

The invention is not limited to the embodiment shown in FIG. 1. Theaqueous amine oxide can not only be injected from the circumference(casing) of the fall zone vertically or angular upward or downward tothe direction of fall, but also at the entry 7 ^(a) of the fall pipeparallel and/or angular to the direction of fall. In the injection fromthe circumference, this is appropriately done in the upper half of thefall pipe 9. The charging speed of the fall pipe is coordinated with theconveying speed of the multiple-shaft apparatus preferably such thatthere is a fill in the lowest part of the fall pipe 7 prior to entryinto the multiple-shaft apparatus 11, so that as little air as possibleis pulled into the multiple-shaft apparatus.

1. A method for the continuous preparation of a suspension of cellulosein an aqueous tertiary amine oxide for use in the Lyocell process, themethod comprising: (a) mixing cellulose and an amine oxide-free aqueousphase to form a cellulose suspension in a mass ratio in the range of 1:3to 1:40, (b) dehydrating the cellulose suspension, wherein thedehydrated cellulose suspension has a cellulose content in the range of20 to 80 percent-by-mass; (c) mixing the cellulose suspension with anaqueous amine oxide, wherein the aqueous amine oxide is injected intothe cellulose suspension while the cellulose suspension is fallingthrough a vertical fall zone that is only partially filled by thecellulose suspension, wherein enough aqueous amine oxide is mixed withthe cellulose suspension to obtain an aqueous amine oxide-cellulosesuspension with an amine oxide content in the range of 70 to 80percent-by-mass; and (d) conveying the aqueous amine oxide-cellulosesuspension through a horizontal shear zone so that the aqueous amineoxide-cellulose suspension fills the available cross-sectionalconveyance area in the shear zone essentially completely.
 2. The methodin accordance with claim 1 whereinN-methylmorpholine-N-oxide-monohydrate is injected into the verticalfall zone.
 3. The method in accordance with claim 1, the amine oxidesuspension dwell time in the shear zone is in the range of 5 to 30minutes.
 4. The method according to claim 1, wherein the cellulosesuspension is dehydrated into a material with a cellulose content in therange of 45–55 percent-by-mass.
 5. A device for continuous preparationof a suspension of cellulose in an aqueous tertiary amine oxide for usein the Lyocell process the device comprising: (a) a mixing apparatuscomprising a mixing container, feed nozzles for introducing celluloseand aqueous suspension agents into a mixing vessel to form a cellulosesuspension and discharge nozzle for removal of the cellulose suspension,(b) a separation apparatus that is communicatively connected to themixing apparatus comprising means to partially separate the suspensionagent from the cellulose to form a dehydrated cellulose suspension; (c)a charge system communicatively connected to the separation apparatuscomprising a vertical fall pipe having at least one nozzle positioned inthe vertical fall pipe for introducing an aqueous amine oxide into thevertical fall pipe for premixing with the dehydrated cellulosesuspension to form an aqueous amine oxide-cellulose suspension; and (d)a shear apparatus communicatively connected to the charge system forreceiving the aqueous amine oxide-cellulose suspension, the shearapparatus comprising horizontal shafts equipped with shear tools and adischarge nozzle for discharge of the amine oxide cellulose suspensionafter passing through the horizontal shafts.
 6. The device in accordancewith claim 5, wherein the at least one nozzle is connected to a loopline for transporting the aqueous amine oxide to the nozzles.
 7. Thedevice in accordance with claim 6, wherein the loop line is connected toa heated supply line.
 8. The device according to claim 5, wherein theshear apparatus comprises a homogenization frame that is shortened inlength relative to a homogenization frame in a shearing apparatus notconnected to a charge system with a vertical fall pipe and chargingnozzles connected therein.
 9. The device according to claim 8, whereinthe shear apparatus is shorter by approximately one third.
 10. Thedevice according to claim 5, further comprising a dissolution apparatusfor receiving the discharged aqueous amine oxide cellulose suspensionfrom the shear apparatus.
 11. The device according to claim 5, whereinthe vertical fall pipe is conically shaped and narrows towards the shearapparatus.